hw/riscv: sifive_u: Switch to use qemu_fdt_setprop_string_array() helper

hw/riscv: sifive_u: Switch to use qemu_fdt_setprop_string_array() helper

Since commit 78da6a1bca22 ("device_tree: add qemu_fdt_setprop_string_array helper"),
we can use the new helper to set the clock name for the ethernet
controller node.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210430071302.1489082-1-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>